1. Field of the Invention
This invention directs itself to waveguide filters for providing cable access to a shielded enclosure. In particular, this invention directs itself to a waveguide filter feed-through for cables previously terminated, and where high frequency attenuation of electromagnetic radiation is required. Still further, this invention directs itself to waveguide filter feed-through devices having a housing in which at least two overlapping passages are formed therethrough, with a longitudinal access opening formed between the two passages. One of the passages, having a smaller diameter, forms a waveguide passage, the other forming a passage through which the terminated ends of cables can pass and into which a plug is inserted to form a closure for the longitudinal access opening between the two passages.
2. Prior Art
Waveguide filters for use as cable feed-throughs are well-known in the art. The best prior art known to the Applicant includes U.S. Pat. Nos. 3,385,970; 4,358,632; 3,587,009; 4,109,222; 4,267,401; 3,851,282; 3,779,585; 3,489,440; 3,534,146; 4,255,616; 4,249,353; 3,092,360; and, 4,693,767.
Some prior art systems such as shown in U.S. Pat. No. 3,385,970 are directed to waveguides operating below cut-off for providing an optical coupling link through a shielded enclosure. The diameter of the central bore and the length of the waveguide are predetermined to achieve the desired attenuation and cut-off frequency. However, when previously terminated cables are used, the bore diameter is no longer determined by the desired cut-off frequency and attenuation, but is mandated by the size of the terminated connector. To compensate somewhat for the increased diameter of the bore, the length of the waveguide must be increased, which in some applications may not be practical due to space limitations. Even with increased length, prior art waveguide filters having bore diameters sufficiently large to accommodate previously terminated cables exhibit lower performance, with respect to high frequency attenuation, than the smaller bore diameter waveguide filter for use with terminated cables.
Thus, as shown in FIG. 1, the prior art waveguide filter feed-through 10 comprises a housing 20 in which is formed one or more waveguide passages 30 extending longitudinally through housing 20. Each of the waveguide passages 30 has a diameter larger than the diameter of the optical cable termination 58 which passes therethrough.
Since the diameter of the waveguide passage 30 greatly affects the cut-off frequency for the device, this diameter is kept as close to the diameter for connector termination 58 as practical. Therefore, only one fiber optic cable 55 can access an individual waveguide passage 30. Thus, a duplex fiber optic cable 50 must be split apart exposing the two individual optical cables 55 and passing each through separate waveguide passages 30, as shown in FIG. 1. In contra distinction, the instant invention provides the means for inserting the previously terminated duplex fiber optic cable 50 through a single waveguide passage, the waveguide passage having a smaller diameter than that required by the prior art, thus providing improved performance, with respect to cut-off frequency and attenuation.
The housing 20 of the prior art device shown in FIG. 1 is provided with threads 15 for coupling to a shielded enclosure with a pair of internally threaded fasteners 90. The instant invention is similarly provided with this method of fastening to a shielded enclosure so as to be interchangeable therewith.
In other prior art systems such as U.S. Pat. No. 4,358,632, there are provided electrically shielded cable entrances for passing cables through a bulkhead. However, these systems are comprised of members split into interfitting halves made from an electrically conductive elastic material which are forced in compressive contact with the cables passing therethrough. These structures provide an electrical grounding type shield, and do not form a waveguide filter.